Yeasts are tiny one-celled organisms that exist naturally in many environments, including on grapes and in wineries. Some winemakers like to ferment their wines with the yeasts that already live in their vineyards or wineries; these are called "native" (also known as wild, indigenous, natural or ambient) yeasts and are believed to emphasize terroir and add complexity to a wine. But native yeasts can result in slow and unpredictable fermentations. Many winemakers inoculate the grape must with "cultured" or commercial yeasts, which are strong, consistent and fast fermenters. There are many kinds of commercial yeasts available, each offering unique characteristics to the finished wine.

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